When AC current flows through the primary winding of a transformer, an alternating magnetic field is generated which induces magnetic flux in the transformer core. The flux has a varying positive and negative polarity and is normally balanced or centered around zero. The flux in a transformer is directly proportional to the integral of the applied voltage and thus its polarity lags the polarity of the voltage by 90 degrees. The rated design voltage of a transformer also implies a rated level of transformer magnetic flux. If the magnetic flux exceeds the rated levels in either the positive or negative direction, the transformer core begins to become “saturated” with flux. This causes the natural impedance of the primary winding of the transformer to become significantly lower than normal, thereby allowing excess levels of current to flow into the transformer. Such current can be ten to twenty times normal full rated current and can result in severe power distribution problems, such as breaker trips, fuses clear, sources become overloaded, etc. This problem can arise, for example when initially energizing a larger transformer in a random or uncontrolled manner. With reference to FIG. 1, if voltage is applied as indicated at 12 at a random point in the sine wave, the magnetic flux 14 is not necessarily centered around zero and maximum allowable flux levels indicated at 15 can be exceeded as indicated at 16 in FIG. 1.
In the context of a static transfer switch, voltage is typically applied to downstream transformer employing a “soft-start” controlled manner which eliminates or minimizes the inrush current. The soft start control turns on the static switch at the appropriate point in the applied voltage waveform (i.e., at the peak) as indicated at 22 in FIG. 2. Since the magnetic flux is proportional to the integral of the applied voltage and assuming the initial state of the magnetic flux in the transformer core to be zero, it can be mathematically shown that this approach guarantees that the magnetic flux in the transformer core is balanced or centered around zero as shown in FIG. 2.
With reference to FIG. 3, if the voltage applied to a transformer is removed at some point in the waveform at which the flux is non-zero, a residual magnetic flux indicated at 32 will remain in the transformer core for a significant period of time before decaying naturally to zero. Depending on the transformer design, the decay time can extend from several minutes to over an hour. Consequently, if a voltage 12 is re-applied to the transformer soon after the voltage has been remover (e.g., after a brief power failure and subsequent recovery), then the “start-soft” control method, which depends on an initial de-magnetized flux state, cannot guarantee to provide the desired results. Rather, the transformer core may become saturated as indicated at 33. Therefore, it is desirable to develop a new method for operating a static transfer switch will eliminate inrush current in the presence of residual magnetic flux.
This section provides background information related to the present disclosure which is not necessarily prior art.